1. Field of the Invention
The invention relates to an apparatus and method for printing upon a recording medium and more particularly to an apparatus for printing permanent images electrographically upon a paper medium at comparatively high speeds as is required in a computer print-out apparatus.
2. Description of the Prior Art
It had long been recognized that computer peripherals, particularly computer print-out apparatus, were bottlenecks in the total performance of a computer system. The majority of hard copy output devices for computer systems were and still are comprised of printers which impact the paper medium with print hammers. The movement of such print hammers not only limits the speed of which read-out can be accomplished, but are noisy and difficult to maintain. In order to increase the speed, facilitate maintenance, and still maintain print quality, a system was developed and is now being marketed commercially by Honeywell Information System Inc. utilizing electrographic techniques to accomplish non-impact printing. Such a printing system is disclosed in the related articles, applications and patents cited supra.
One main problem in the design of such an electrographic printing system arising at the toner station where the latent electrographic image is developed, i.e., made visible by subjecting the paper medium to charged toning particles suspended in liquid toning carrier. The problem is to increase the variable print densities greater than a reflective print density of 0.85 yet not substantially increase the speed of the paper medium in excess of 30 inches per second. There are several methods of attack to this problem. Referring to FIG. 1 some of these approaches will be discussed. FIG. 1 shows a toning station 100 which is comprised of a toning reservoir 105 containing toning liquid 106. Toning liquid is comprised of positive charged toner particles which are dispersed in a liquid carrier typically Isopar-L (a trademark of Exxon Corporation). A paper medium 103 pre-treated with dielectric 104 travels to the toner station in the direction of arrow 107. The dielectric face 104 has negative electrical charges in a pattern determined by previous operations thereon as described in the above-related references and in particular references 1-7. At the toner station a transfer roll comprised of any suitable conductive materials such as wear resistance steel is partially submerged in the toner liquid 106 and rotates counter clockwise at a constant speed and picks up toner liquid 106 and carries it to the meniscus 102 formed by the gap between the transfer roll 101 and the dielectric coded paper 103, 104. Since there are negative charges in a selected pattern on the dielectric 104 of the paper 103, the positively charged toner particles suspended in the meniscus 102 will travel to the dielectric surface 107 where they will be captured by the negative charges thereon and adhere thereto. It is obvious that the more toner particles deposited on the dielectric surface 104, the greater will be the print density. One way of depositing more toner particles is to decrease the gap between the tranfer roll 101 and the dielectric surface 104. Since the field E from the grounded transfer roll 101 to the dielectric surface 107 is inversely proportional to the gap between the transfer roll and the dielectric coded paper (i.e., the closer the transfer roll to the dielectric surface the greater the field) more particles would be deposited on the paper. However, by making the gap between the transfer roll 101 and the dielectric surface 104 smaller, the meniscus 102 is reduced and accordingly will hold less toner particles. Hence this would tend to decrease the print density. This could be remedied somewhat by increasing the speed of the transfer roll 101 but there is a maximum rotational speed which is determined by the centrifugal force at the outer rim of the transfer roll 101. The greater the speed, the greater will be the centrifugal force at the rim of the transfer roll 101 and would cause splattering of the toner mix onto the dielectric surface 104 which is undesirable. Another way of increasing the print density is to make the toner meniscus 102 longer; hence it will have more toner particles and also the dielectric coded paper 103 will remain in the meniscus for a longer period of time. One way to make the meniscus 102 longer is to make the transfer roll 101 larger. However, this would require a deeper reservoir and the slowing down of the revolutions per minute of the roll in order not to exceed the peripheral speed; some form of gear reduction would be necessary. Moreover, there is a limit to increasing the size of the meniscus using this technique.
Another major problem in developing the latent electrographic image is print contrast. This problem is at the opposite pole of the previous problem in that what is required is that the background density i.e., the density of the toner particles on the portion of the paper not selectively charged, should be kept as low as possible. Yet another problem related to the second problem is that of background staining of the paper as it passes through the transport. These and other problems encountered do not offer trivial solutions in developing the latent electrographic image with clarity, high print density, low background density of toner particles, and minimization of background staining.